As a conventional optical transfer device for transferring a multi-channel signal through an optical fiber, there has been proposed an optical transfer device which modulates the multi-channel signal by a modulator using an optical heterodyne detecting system and converts the signal thus modulated into an optical signal to be transferred (For example, see Non-Patent Reference 1).
FIG. 5 is an arrangement view of a conventional optical transfer device, including an FM modulator using the optical heterodyne detecting system. The conventional optical transfer device includes an optical transmitter 11 constructed of an FM modulator 12 for modulating an input multi-channel signal into an FM signal which is an electric signal and an EO converter section for converting the FM signal into an optical signal to be sent out to an optical fiber 4; and an optical receiver 5 constructed of an OE converter section 6 for converting the optical signal transferred through the optical fiber 4 into the electric signal and an FM demodulator 7 for FM demodulating the electric signal to be outputted.
Now referring to FIG. 5 and FIG. 6 showing the output spectra of the respective components of the conventional FM modulator and a conventional FM de-modulator, a detailed explanation will be given of the FM modulator 12.
A branching circuit 121, when a multi-channel signal frequency-multiplexed as shown in FIG. 6(a) is supplied to the FM modulator, supplies two multi-channel signals phase-converted to a first semiconductor laser 122a and a second semiconductor laser 122b which provide different wavelengths, respectively.
The semiconductor laser 122a and the semiconductor 122b supply, to a photo-coupler 123, FM-modulated light beams as shown in FIGS. 6(b) and 6(c) whose optical frequencies are FM-modulated on the basis of the multi-channel signal received, respectively. The spectrum of each the FM-modulated light beams produced from the semiconductor lasers 122a, 122b is FM-modulated in the vicinity of about 200 THz by the multi-channel signals supplied to the semiconductor lasers 122a, 122b (The spectrum is spread by the FM modulation). Further, when the optical frequencies of the semiconductor lasers 122a, 122b are FM-modulated, their strengths are also modulated so that the spectrum of the FM-modulated light from each of the semiconductor lasers 122a, 122b generates a residual AM signal having the same frequency component as that of the multi-channel signal. The residual AM signal generates noise in the multi-channel signal after FM demodulation and deteriorates the distortion characteristic.
A photo-coupler 123 merges these two FM-modulated light beams and supplies the resultant optical signal to a light-receiving element 125. Since the two multi-channel signals which FM-modulate the optical frequencies of the semiconductor laser 122a and the semiconductor laser 122b are inverted in their phase, the above residual AM components are also converted in their phase. Thus, in merging, the photo-coupler 124 cancels the residual AM components the residual AM components of the FM modulated light beams produced from the semiconductor laser 122a and the semiconductor laser 122b, respectively. As a result, the optical signal produced from the photo-coupler 124 does not contain the residual AM signal as shown in FIG. 6(d).
The light receiving element 125 produces, through optical heterodyne detection, the FM signal which is an electric signal based on a frequency difference between the two FM-modulated light beams thus merged. FIG. 6(e) shows the FM signal produced from the light receiving element 125. This FM signal is acquired by FM-modulating the multi-channel signal supplied to the FM modulator 12. Its central frequency is determined by a difference between the central values of the optical frequencies of the two semiconductor lasers 122a, 122b. 
The optical receiver 5 receives the multi-channel signal as shown in FIG. 6(f) obtained by FM-demodulating the FM signal shown in FIG. 6(e).
In this way, in accordance with the conventional optical transfer device including the FM modulator, by modulating the optical frequencies of the two semiconductor lasers on the basis of two multi-channel signals with inverted phases, respectively and merging the FM-modulated light beams produced from the two semiconductor lasers, the residual AM signals contained in the respective FM-modulated light beams can be cancelled. For this reason, the multi-channel signal supplied to the FM modulator can be modulated with the residual AM signal suppressed (For example, see Patent Reference 1).
Further, an explanation will be given of the optical phase modulator not generating the above residual AM signal referring to FIG. 7, which is a configuration view of a conventional optical transfer device including an optical phase modulator using an optical heterodyne detecting system, and FIG. 8 showing output spectra of the respective components of a conventional optical phase modulator and a conventional FM demodulator. In FIGS. 7 and 8, like reference numerals refer to like elements in FIGS. 5 and 6.
Semiconductor lasers 222a, 222b produce optical signals with no modulation (local light beams) shown in FIG. 8(b).
An optical phase modulator 223 phase-modulates the local light beam produced from the semiconductor laser 222b on the basis of the frequency-multiplexed multi-channel signal, thereby producing the phase-modulated light beam shown in FIG. 8(c). The spectrum of the phase-modulated light beam produced from the optical phase modulator 223 is modulated in the vicinity of about 200 THz by the multi-channel signal supplied to the optical phase modulator 223. By phase-modulating the local light beam produced from the semiconductor laser by the optical phase modulator 223, the residual AM signal is not generated.
The photo-coupler 223 merges the local light beam produced from the semiconductor laser 222a and the phase-modulated light beam produced from the optical phase modulator 223, thus supplying the optical signal shown in FIG. 8(d) to the light receiving element 223.
The light receiving element 225 produces, through optical heterodyne detection, a phase-modulated signal which is an electric signal based on a frequency difference between the merged two optical signals. FIG. 8(e) shows the phase-modulated signal produced from the light receiving element 225. This phase-modulated signal is acquired by phase-modulating the multi-channel signal supplied to the optical phase modulator 223. Its central frequency is determined by a difference between the central values of the optical frequencies produced from the two semiconductor lasers 122a, 122b. 
The optical receiver 5 receives the multi-channel signal as shown in FIG. 8(f) obtained by FM-demodulating the phase-modulated signal shown in FIG. 8(e). Since the phase modulation and the frequency modulation are angular modulation systems belonging to substantially the same category, the multi-channel signal can be demodulated by the FM demodulator 7 in the optical receiver 5.
In this way, in accordance with the conventional optical transfer device including the optical phase modulator, by phase-modulating the local light beam produced from the semiconductor laser on the basis of the multi-channel signal, the multi-channel signal supplied to the phase modulator can be phase-modulated without generating the residual AM signal (For example, see Patent Reference 2).
Patent Reference 1: JP-A-11-112433
Patent Reference 2: JP-A-10-13354
Non-Patent Reference 1: K. Kikushima, et al.: Super-wide-band optical FM modulation scheme and its application to multi-channel AM vide transmission systems, IEEE Photonics Technology Letters, pp. 839-841, 1996.